I know I've gone on and on about IPTC.  And all over the map.  But people are still looking for the information and even I can't find my way through the old maze anymore.  For this reason, I will now attempt to consolidate some options into one easy-to-access summary.

Picasa.  Free organizer.  I never use it but I keep it around because it's so darn cute.  It works, for both IPTC keywords and captions, on jpg's.  Putting captions and keywords on anything other than jpg's here is a waste of your time. You can write keywords and captions to tiff's and psd's but it's proprietary, i.e. you will not see this information in IPTC fields in other software.  If you have IPTC info entered on tiff's and psd's in other software it will not show up in Picasa.  There are no other information fields so it's treding on the outer edges of what IPTC can be.  Nowhere in this software does it even say the word IPTC but that's what it is.  Batch mode works by selecting a range of thumbnails, clicking Ctrl+K to open the keyword box and adding your keywords.  "Search" will bring up thumbnails for file and folder names (including all sub-folders) as well as keywords but it does narrow it down somewhat.

Photo Info.  Free.  Not an organizer, but a Microsoft add-on that sits in your context menu and writes IPTC to jpg's and tiff's and a few exotic file formats.  Has a range of fields to cover most anything you could want.  Also has a batch mode.  A little slow and sticky in its execution but worth having.  You'll still need something else that can read and search IPTC on a mass level.

XnView.  Free photo viewer.  Can read IPTC info on psd's, tiff's and jpg's but will only write IPTC to jpg's.  Captions, keywords, copyright, byline, status and date.  More thorough than Picasa if you only work with jpg's.  Can also batch edit IPTC and search all IPTC fields, (including the non-jpg's).  Very nice program.

iTag.  A nice little program for adding captions and keywords to jpg's only.

I have looked at several other free programs that claim to write IPTC-info.  When it comes down to it, they only write to jpg's.  There's probably a hundred of that caliber.  If the choices here don't suit you, search Google and take your pick.

Windows File Properties is not IPTC.  Although it shows EXIF information, the summary fields available for adding information are not EXIF either.  They're not anything except Windows File Properties.  I do not recommend this as a way to annotate files unless you're a masochist.  

FastStone Viewer.  Free.  Does not read or write IPTC.  If you change and re-save photos here sometimes it will strip out any embedded-IPTC info because it doesn't recognize its relevance.  Sometimes it leaves it in.  You'd have to experiment to know which is which.  

iMatch touts itself as a professional-quality image-management system, including IPTC.   In my humble opinion,  it's a train-wreck.  If you don't believe me, you can try it free for 30 days.

ACDSee Pro 2 Photo Manager. Editor, Organizer, and general Photo Wizard.  Reads IPTC on psd's, tiff's and jpg's.  Writes to tiff's and jpg's.  It has detailed batch modes for anything you could possibly need.  It can also catalog text documents, pdf's, etc.  I like it alot, although I haven't so far been able to figure out the search function.  I'm sure it works because everything else does. (20% off coupon code 2PRO20 til June 30th.)  Suitable for anyone, beginner to advanced.  1 GB of RAM recommended.  There's a lesser version called 10 Photo Manager although I haven't worked with that one.  From what I've read it seems the keywords are IPTC, the categories belong to the ACDSee database itself.  (20% off coupon code 10PM20 until June 30th)

(April 1st update: The special ACDSee prices on these two products are being extended indefinitely.)

Adobe Photoshop Elements.  Editor and Organizer.  Can read and write to psd's, tiff's and jpg's.  (Nothing writes to bmp's.)  If you add captions, copyrights, etc in the Editor it will save when you save changes to the photos.  If you add tags and captions using the Organizer you have to remember to use "Write Tag and Properties Info to Photos" under File on the main menu, to make it stick.  Does not have a batch mode for anything other than 'tags' (their word for keywords) but does have Search options.

Adobe Photoshop Lightroom.  Editor and Organizer.  A more mature presentation if Adobe Elements insults your intelligence.  Video tour available and the usual free trial.

MediaDex.  Organizer.  Reads, writes and searches IPTC in any file format, with a virtually unlimited range of fields.  Although not recommended as a beginner's program, anybody with fair-to-middling experience and some patience can work with this.  Tech support is limited to one primary responder at their forum and a fairly unintelligible manual.  It can be maddeningly unintuitive and utterly useless in places but its depth and range of options makes it very attractive.  Love it or hate it.  It has immense potential and I'm praying for a Version 3.0.  It took me about a month to hit a comfort zone with it.

There are others such as Photo Mechanic and BreezeBrowser, dedicated IPTC-er's rather than other things. Photo Mechanic is fine-dining.  If nothing else, go taste it.  Of the three I've spent the most time with, Adobe, ACDSee and MediaDex, I'd have a hard time picking just one.  I like Adobe as an editor, MediaDex for its killer search engine, and ACDSee Pro 2 for its detailed batch processing.  All of the retail products have trial versions and that's the best way to find the one to suit you.

Legacy Family Tree, Passage Express and JAlbum will import IPTC captions from jpg's and uncompressed tiff's.  Passage Express acts somewhat erratically with this, although I still don't know why.

If you allow Windows to resize your photos when you email them it will strip out any IPTC-embedded info, so send your photos at their original sizes.  This is not a failure of IPTC, it's just the idiocy of Windows.  If you need more space than email allows, sign up for a free YouSendIt account. (under the Sign Up tab)