|An example pattern from Colourlovers|
The first site is STRIPE GENERATOR and you can use it to make your own perfect, custom striped patterns. Easy peasy!
Also by the same team comes TARTANMAKER. Both sites allow you to add and remove colours and adjust various parameters (such as shadow, angle, width...) all with an intuitive slider system. Once you're finished you can download your pattern and if you wish, can create an account to save your patterns on the site.
Another pattern generator site that I've bookmarked is JAPONIZER. As the name suggests, this site will let you customise traditional Japanese patterns, again all seamlessly tileable. Ignore the large DOWNLOAD button on the page (it's an ad!), just hit JAPONIZE once you've finished tweaking your design to download it.
The final site I'd like to recommend is one I just discovered today. COLOURLOVERS allows you to customise a massive array of shapes, colours, and abstract designs and save them to share with the community. You can download any patterns you like at the right hand side of the "Browse Patterns" pages. I'd recommend downloading the preview size, then you can tile it in your graphics software. There are lots of other cool features to this site, such as swathes of colour swatch palettes for inspiration as well as various social networking features and trend lists.
UPDATE (19/3/12): I just came across another fantastic online seamless pattern generator called Pattern Cooler.