Nature: I know, people say this all the time but its true! Take a walk in the park or in the woods depending on where you live, take it in, look at stuff, textures, shapes, contrast, colours, it's all there!
Architecture: This is easier if you live in the city. Go look at buildings. Old ones, new ones, the lines, the patterns, the layouts etc.
Now on to links:
CSS Garden - http://www.csszengarden.com Choose a design on the right. Basically this website is done in CSS and designers are challenged to redesign it completely using only CSS, each of these designs are available to be viewed, go through a couple and see what you like!
http://kuler.adobe.com - This is an awesome wesbite/community for sharing colour swatches. There are tons of different sets, and their tagged so you can search for a theme and perhaps find colours that match it. This is really awesome if you dont know what colours to use
http://www.behr.com/behrx/workbook/ - This is also great for colour. Choose a few colours and the rest will be calculated to match. This is a tool for designing rooms but it's awesome for website colour schemes too. (click "START COLORSMART")
http://www.deviantart.com - Awesome art community. Consider browsing through some of the sections that might be more relevant to what you're designing at the moment.
http://smashingmagazine.com Awesome. See for yourself. This website is the epitemy of useful.
Those are the ones I use most often, what are yours?
My desk also works well for inspiration! Its always untidy and just covered in random books, CDs, DVDs, leaflets and brochures. Sometimes a combination of shapes or colours just catches my eye and I end up using it to design something. Obviously the disadvantage to this is a very untidy desk 
