A Short Syntax of New Testament Greek - Nunn - Actually pretty useful for reference.
Selections from the Septuagint, Conybeare and Stock - Definitely worth downloading! The grammar section is very helpful for understanding the transition from classical > LXX > NT. Some fun LXX texts to read with nice introductions and notes.
Greek Grammar, Herbert Weir Smyth - This is a classical Greek grammar, but the better Greek in the NT is understandable with reference to this grammar.
Syntax of the Moods and Tenses of the Greek Verb, William W. Goodwin - Again, a classical Greek reference. The Wallace book is better for NT studies, but hey, this download is free.
Pocket Lexicon of Greek New Testament, Alexander Souter - If you need a little lexicon, this is fine.
NT Greek in a Nutshell, James Strong - Some paradigms, etc. Might be helpful for analysis of some of the texts it treats.
The Gospel of St. Luke in Greek, H.R. Heatley - Maybe a few helpful notes. (Look in the back part of the book.)