Shoutout to InfoPathDev.com - several of our product team alums are hosting an excellent resource for the InfoPath developer community. Lots of great content there: sample form templates, very lively forums, and walkthroughs. My personal favorites:
1) Localizing form templates. Very clever method, using secondary data connections to store localizations in XML files. I still refer to it as "the Shiraz Cupala method". Note that this method works even better with InfoPath 2007, as button labels can now be dynamic (can come from a data source).
2) Super-FAQ: auto-generating sequential row numbers.
3) Interop between rules and code: article on xdextension. Note that this trick is not supported in browser-enabled form templates.