Sheimos HaTzaddikim is a very unique prayer within Breslov which is pretty much reciting the names of many righteous people, from characters within the Tanach to fairly modern figures.
I could not find an informative page for those who haven't heard of the prayer, but I did find the text of the whole thing on ויקיטקסט, which you can see here.
The Breslov Siddur says the following:
The very names of the tzaddikim possess a profound mystical significance. Rebbe Nachman's teachings on this subject prompted Reb Noson to compile the Sheimos HaTzaddikim (Names of the Tzaddikim), which includes all the tzaddikim in the Bible and in rabbinic literature.
There are many, many, figures in this compilation of which I am not familiar with. Is there a book or somewhere online in which I could look up the name of a tzaddik and read a short biography?
Such an encyclopedia would be useful even for those not reciting Sheimos HaTzaddikim, as sometimes you might read about a Tanna, Rishon, etc. you don't know who is.