Outsourcing is big business and Outsourcing Advisory firms have a very important role to play, Forrester has a presentation with an overview of some of the firms, listed below:

Deloitte
EquaTerra
neoIT
PA Consulting Group
Pace Harmon
PricewaterhouseCoopers
RampRate
TPI

The have a huge commercial impact as the IT and Application outsourcing market is around 120 Billion $ per year & $500,000,000,000 in TCV, A conservative estimate is that outsourcing agreements lock up more than half-trillion dollars of capital in total contract value (TCV).

Also
1. Sourcing deals are being restructured
2. Selective sourcing deals are growing in popularity
3. Several markets have latent growth opportunities.

The presentation also has two charts with the outsourcing advisory firms profile and protfolio, charts below:

Source: 1

Prashanth Rai