Color Schemer Online is, in my opinion, the best online tool for generating schemes. I like it because I can lighten or darken the scheme to get many complimentary colors for my project.
Another really cool site is ColorLab Here you can learn color theory basics, what colors mean, and color harmony. Their generator creates 7 different schemes: Monochromatic, Analogous, Complementary, Split complementary, Triad, Tetradic 1, and Tetradic 2. If you don't like the scheme you've generated, you can use their online color picker to select another color and generate more schemes. You can play with this tool until you get the palette you want. The added bonus on this site is their web page generator. You can change the colors on various elements and when you click submit, a valid XHTML two-column fixed-width template page is created for you. How convenient is that? I think it's very nice, especially for a newbie web designer.
ColorMatch Redux is another nifty tool that lets you export the generated scheme to an .ACT file for use in Photoshop or Fireworks, an .AI file for Illustrator, or a text file for any other program. Simply adjust the slider or enter the RGB values of your color and you'll have a nice nine-color scheme.
If you have an image that you want to use on your website, you can upload the image at colr.org (http://www.colr.org/), and then select colors or schemes that would compliment your image. This is really handy especially if your image is a main element of your web page. What's also neat is that you can create tags for your scheme and save the palette so you can come back any time to re-use your palette. Let's say you want a palette that represents a flower. Enter "flower" in the tag search box, and all the palettes tagged as "flower" will load, allowing you to choose one of the existing schemes.
By using any one of the online tools above, your site's colors will never clash, and you've saved yourself a lot of time trying to figure out which colors to use on your project. Now go play with colors!