Apple's latest $29 update to OS X Snow Leopard ships this Friday--but will all of your essential applications run when you install it? This application compatibility list is a good place to find out. I've been running various builds of the Snow Leopard preview for two months now, and every single one of my most important applications (with the exception of one*) is working just fine.
* I happen to use an abandoned beta of Synergy KM to share my mouse and keyboard between my PC and Mac, and its System Preferences pane requires a restart every once in awhile to get it to connect. Overall, a minor annoyance for other worthy updates.
Notable non-compatible apps to look out for from the list: Adobe CS2 Suite, Adobe Photoshop Elements, CoverSutra, Cyberduck, (maybe) Disk Inventory X, Disk Warrior, and (maybe) Google Gears. (Google Gears occasionally does this weird thing where it launches a virtual disk and then closes it again; otherwise it has worked for me. Disk Inventory X also works for me, albeit slowly, in build 10A421a of Snow Leopard.)