At Muxtape, you can upload up to 12 songs, but you can have no more than one song from any album and you can have only one mixtape per user. Mixwit is a bit fancier in that you can upload your own pictures to use as a skin for your tape in addition to the styles on the site. Both Muxtape and Mixwit join the existing iTape MP3 player type on MyFlashFetish, which lets you add up to 100 songs.
As with physical hardcopy mixtapes, you can share your online creations. You get your own URL with Muxtape and Mixwit, and with Mixwit and MyFlashFetish you can embed code onto social networking sites and blogs. Another approach is that you can generate your own cassette image on Says-It.com and have that be the icon for your playlist on finetune. On finetune the playlist has to have at least 45 songs, can have no more than 3 songs per artist, and consists of songs from finetune’s library. But the library is big enough such that you can find a good percentage of what you are looking for. With finetune you get both a direct URL and the ability to post outside of finetune. Actually this last approach isn’t an official mixtape service per se, it’s just something that I’ve done personally!