Firefox: Mozilla's been cranking away at their experimental (but seemingly stable) Contacts add-on, making it a really convenient one-stop shop for getting to all your Gmail, Twitter, LinkedIn, local address book, and now Yahoo and Facebook contacts in one searchable place.
The two big changes to Contacts for the 0.3 version are the inclusion of Facebook contacts, made possible through the recent Facebook "Social Graph" API, and your Yahoo address book. Aside from more connectors though, Contacts now lets you simply start typing a person's name into the address/AwesomeBar, and names from your Contacts database will show up there. So, rather than heading to Gmail and discovering that Steve Smith is actually your Facebook friend, you can just type Steve Smith into your Firefox bar, his contact will pop up, and you can get at Steve Smith however you'd like from there.
Contacts is a free download that should work wherever Firefox does. Got a killer use for Contacts? Do let us know in the comments.