Ten days ago I mentioned a paper by Zhiyong Lu that gives an overview over the available web tools to search the biomedical literature. Most of these tools enhance the PubMed service, and Zhiyong Lu in fact works for the NCBI, the developer of PubMed. In this post I want to take a more detailed look at the available tools. A good starting point is the companion webpage to the paper, listing 28 services.