For a long time I've been looking for a good diagramming/vector-based drawing program that meets my needs as a developer. I'd like to:

1. Draw database diagrams
2. Draw flow charts
3. Draw object-modeling diagrams (UML being the standard)
4. Draw other free-form diagrams (basically boxes & arrows with the occasional clipart)
5. Draw mockups of user interfaces and web pages

EDIT: I want good-looking electronic-format diagrams that I can show to 3rd parties, not just something for my own internal use.

EDIT 2: I'm also looking for Windows software, although I'm toying with the idea of switching to Mac, so a really good Mac-only product might get me to switch.

Basically I need a good vector graphic program (with decent grouping, connecting lines, and ideally auto-routing). I'd prefer a diagramming tool that can also be used for drawing (for the UI mockups) rather than a drawing tool that can also be used for diagrams.

I've tried Visio on several occasions, and every time I've been disappointed. The interface always seems to get in my way at some point. It's pretty close to what I want, and the latest version (I got the trail from MS) seems to be better than previous ones in terms of usability, but I really don't want to plunk down that sort of cash for a mediocre product.

I've tried Dia and Inkscape, and while initially promising and with the right price tag, I found both of them to be lacking in several ways (including some recurring bugs).

I've toyed with getting Adobe Illustrator, but I've never used it before, and I have a feeling that it wouldn't handle the diagramming aspect very well, and I don't want to buy a copy just to find out it doesn't meet my needs.

So far, the product that I've had the most success with is, sadly, OpenOffice Draw. It's free of course (which lowers my expectations and thus improves my view of it) and its usability is pretty good, but in the end I'd like something more suited to diagramming.

I'm willing to spend real money (in the $500-$1K range) for a really good piece of software if it does everything I want it to. The front runner is of course Visio but I'm hoping for more. Does anybody have any recommendations?

CONCLUSION: @dlamblin had the most informative post, but the part I gained the most from was his/her (and others) mention of OmniGraffle, not Gliffy.

I gave Gliffy a try, and it seemed neet for occational use, but since it's a Flash app (note: not AJAX as dlamblin mentioned) it's still a bit of a pain to use (no keyboard shortcuts for copy/paste was pretty much a deal breaker for me).

I also tried SmartDraw, but it had 3-strikes-you're-out against it:

1. The trial period was only 7 days long.
2. It used some nonstandard (and visually jarring) GUI widget toolkit for its UI. At the very least it makes me suspicious (how do I know it will actually work & support the standard Windows features?)
3. It crashed on me early into my trial.

OmniGraffle looks like exactly what I want... except that it's Mac-only (so I couldn't give it a try). However, it got good reviews from my Mac-owning coworker, and I hope to try it on a friend's Mac soon. If it's good enough then I might spring for a new MacBook.