Aside from sourcing/category management -- perhaps best defined on an outsourced basis as category lifecycle management -- in the procurement BPO sphere, another rapidly growing segment is supplier management BPO. The best-known vendor in this area is barely known at all, in fact. Achilles is likely the largest procurement BPO you've never heard of (if you're in North America especially, where they have but a small operational presence and no sales and marketing to speak of).
Achilles and other providers in this area (e.g., the procurement and enterprise risk management practice of Deloitte) specialize in delivering BPO offerings for supplier qualification, on-boarding, risk management, performance management, auditing and general compliance. While the specific capabilities of different providers varies, Achilles focuses on delivering these capabilities as a shared service to an industry consortium where suppliers typically pay a single time to "be managed" by Achilles, which delivers the resources and software to collect, manage and analyze the information that organizations require (e.g., compliance-specific supplier data for oil and gas and utilities companies).
Including the management of EHS/HSE (environmental health and safety or health safety and environment, if you're curious) information for a supply chain, Spend Matters estimates the supplier management BPO market at over $200MM per year. This is a market that is growing as fast or faster than any other procurement BPO market segment. Yet you won't find many of the big Indian players (e.g., Infosys, Genpact, Wipro) or even the Western firms doing much of interest here in practice, at least not so far (don't be sold on PowerPoint capabilities or benchmarks, especially with offshore providers). The real and sustained client success stories in supplier management BPO are coming from specialist providers that understand and specialize in this segment of the market.