Here are the resources I have found absolutely the most useful while designing for clients. Listed in descending order:

1. Dribbble

This invite-only “show and tell for designers” site has bar-none the highest quality of UI work online. You can find great examples of almost anything here.

Here are a few more people for you to follow as well:

• Victor Erixon. Has a very distinct personal style– and it’s great. Beautiful, clean, flat designs. Dude’s been a designer for like 3 years and is already top of the game.
• Focus Lab. These guys are “Dribbble celebrities”, and their work lives up to the reputation. Really diverse; always top-notch.
• Cosmin Capitanu. An awesome wildcard. He makes things that look crazy-futuristic without being garish. Really good with colors. He doesn’t really focus on UX though— which is also a criticism of dribbble at large.

2. Flat UI Pinboard

I haven’t the slightest idea who “warmarc” is, but his pinboard of phone UI had been ridiculously useful to me trying to find disparate examples of beautiful UI.

3. Pttrns

A directory of mobile app screenshots. The nice thing about Pttrns is the whole site is organized by— wait for it— UX patterns. This makes it very nice to quickly research whatever piece of interface you’re currently working on, be it a login page, a user profile, search results, etc.