Need to start some were in modding right? Why not try the Half-Life modding Kit. This Kit is designed to have all the tools you need to start your own Half-Life mod, simple mods can be made or advance ones like Base Defense with this small set of tools. If you want to start modding Half-Life you should try this.
Just for the people who are only interested in the coding aspect of Half-Life Modding Kit. This version includes only the Official Half-Life SDK's including SDK 2.1, SDK 2.2, and sdk 2.3. This also includes the Half-Life Summons SDK made off of the sdk 2.3