The Proliant server family was initially manufactured by Compaq. After HP's buyout of Compaq, they have continued production. It is a highly robust family of enterprise-grade servers. They are also widely available on the used market, with Generation I and II (currently up to G-V) machines being available for under $300, sometimes under $100.
I personally run many Generation I Proliants including models DL-380, ML-320, ML-360, and others.