An early mechanism for the distribution of useful algorithm implementations was CALGO, the Collected Algorithms of the ACM. It first appeared in Communications of the ACM in 1960, covering such famous algorithms as Floyd's linear-time build heap algorithm. More recently, it has been the province of the ACM Transactions on Mathematical Software. Each algorithm/implementation is described in a brief journal article, with the implementation validated and collected. These implementations are maintained at http://www.acm.org/calgo/ and at Netlib.
Over 750 algorithms have appeared to date. Most of the codes are in Fortran and are relevant to numerical computing, although several interesting combinatorial algorithms have slithered their way into CALGO. Since the implementations have been refereed, they are presumably more reliable than most readily available software.