I'm thinking of building a site that aggregates a bunch of content from other sites, only displays an excerpt, and links back to the original site. Is there any copyright issues with doing this? Wondering how sites like theawesomer.com and others do it.
If the only content on your site is excerpts off other sites, it is seen as copyrights infringement. Add text and other stuff from your own so that it does not look like just a collage made up of different sites. Another thing to do would be the use of canonical tag if it suits you in the condition.
Would it even be copyright infringement if I am linking back to the site? How does www.alltop.com do it?
Just take a look at alltop.com/copyright/ to see what they do. However, a lot of sites do syndication of content in this way and link back to the original article. So, it's really down to the individual content owner if they want it there or not. If you have a mechanism whereby they can request removal, then you should be OK. Just make sure you're not scraping web pages and instead using RSS feeds.
There is a plugin, guest blog on Wordpress, which fatches articles from EzineArticles..... this way you can find some similar plugin too.