UNCTAD has published the results of an analysis of activity in 2009. The report, Latest Developments in Investor-State Dispute Settlement, IIA Issues Note No. 1 (2010) (UNCTAD/WEB/DIAE/IA/2010/3), is made freely available as a pdf. The earlier report on BITS 1959-1999 is also available to download free.
And just to pick out for special mention two items which are deservedly popular among researchers from the riches of the UNCTAD site you will find on their webpages freely searchable databases of Bilateral Investment Treaties
and of Double Taxation Treaties