Do not exist; Twitter marks all of their links that are outbound as no follow. Even the links in Twitter user’s bio, but that does not mean Twitter cannot be used to bring in links.
The Twitter feed known as Twylah, pulls all tweets from your Twitter stream and post them on your profile page. The only problem with this is that Twitter users will most likely be using URL shorteners which such as bit.ly, this means that the URL will not have an anchor text, which would be very helpful, however having that link which points to your site is better than nothing.
If your Twitter profile is on topic the majority of the time, then the links that are sent out will be seen as more important. Not only that, but if you do some link building to your Twylah page to increase its’ PR and SEO value, then the links that Twylah send out will become even more important!
If you want to make the most out of Twylahs links, stop using URL shorteners as this will help with branding. However if your posts are too long, then you will either have to live with bit.ly’s or shorten the rest of your content.
You must not get too carried away with this using Twitter as a link building service, if you would like to do this then I would suggest creating a dummy profile just for SEO links. Because if your following on Twitter joined before you started using Twitter as a link builder, you will no doubt lose your following.
Twylah is a great way to pull in free links, but it is essential to not get carried away with it, make sure your original use for Twitter is still there, otherwise you will find your profile is losing follows gradually until you have none left.