There are some good resources mentioned already on this site. In particular see this question: http://mathoverflow.net/questions/2147/most-helpful-math-resources-on-the-web

Ryan Budney makes a comment under Thanos D. Papaïoannou's response about MathSciNet. Given any relevant paper on the topic of interest, you can find papers that cite it.

This question may also be worth looking at: http://mathoverflow.net/questions/1722/free-high-quality-mathematical-writing-online