I once read something by a writer who said he doesn't believe in writer's block. It made sense to me - I can always write something, even if it's just a long whiny complaint to my friends about how I can't write worth a damn.

What it really means is that I can't write what I wanted to write, and how I deal with it depends on why I'm stuck. I have a whole list of methods that I heard about from some writing panels I went to. If you want more suggestions, I can dig them up for you, but here are a few of the things I've done that have worked at different times:
Put the pen down for a while and just try to visualise the scene you're trying to write. I find that the more clearly I can imagine the scene, the easier it is to write it.
You could try an outline. I've heard there are some strange people who like to write from an outline. It helps them organise their thoughts or something.

Now me, I'm *very* non-linear. If I get stuck on something, I generally don't worry too much about trying to slog through it. I just skip to another scene, or another chapter, or another story altogether. I can almost always think of something to write... it just might not be what I intended to write when I picked up the pen and paper. If you haven't figured it out yet, try to find out which method works for you, and don't worry about how people say you're *supposed* to do it. They're all wrong.
Get bored. I know, it sounds weird. But seriously, boredom is probably the single greatest source of inspiration I have. Just try committing yourself to sitting down for an hour with a blank piece of paper and a pen, and no other distractions. If you can go for that entire hour without writing something, you are a far more patient person than I am.
The tried and true method which a couple of people mentioned already: just write. It might sound counter-intuitive, but sometimes even writing something that sucks can get the ideas (hopefully better ones) flowing. I find that the more I write, the more I want to write.
Some people like to write in isolation and don't want to talk about their writing. If you're not one of those people, you could try brainstorming. I have a couple of friends who help me a lot with my writing. I love brainstorming with them, bouncing ideas off of them, and getting ideas from them in return. Sometimes an offhand comment will lead to a really great scene.
And now for my favourite way to get un-stuck... Something that you see a lot in the fanfiction community is writing challenges. The challenger will usually set out certain elements that have to be included in the story, maybe one or two, maybe a whole list depending on how elaborate the challenge is. It's a lot of fun, and for some people it's a great way to get the creative ideas flowing. I found that when I started writing in my own 'verse, it worked well for me. When I say to one of my friends, 'Give me a word!' they know I'm stuck and need a little help. If you want to try this one, you might want to pick your helpers carefully. It can be difficult to work words like 'aardvark', 'defenestration' and 'sesquipedalian' into your story about gangsters with magical powers in Southern Ontario.