Evobot is a metamod plugin for the original Natural Selection mod for HL1. It provides AI players to fill out online listen or dedicated servers, or allow for single-player experiences entirely offline. It supports NS 3.2 and 3.3, and all gameplay features (including marines, aliens and the commander), across both regular and combat game modes.
This is the full release of v1.21 of Evobot. Download this if you do not already have Evobot (full instructions in the readme), or if you're running a version older than 1.2.