Most commentaries render this as some sort of thorny bush (see Rashi, Ibn Ezra).

In terms of identifying which exact species it may have been, a great resource is Rabbi Kaplan's Living Torah commentary to [Exodus 3:2][1], which states:

>S'neh in Hebrew. This is most probably the black raspberry (rubus sanctus), which has berries that turn red and then black (Yerushalmi, Ma'asroth 1:2, 3a; cf. Septuagint; Vulgate). The Midrash also identifies it as a species of thorn-bush (Sh'moth Rabbah 1:9, 2:9). Others identify it as the sana plant (Casia obovata), the shurbu (Colutea istria) of the wild juju (Zizyphus spina).


  [1]: http://bible.ort.org/books/pentd2.asp?ACTION=displaypage&BOOK=2&CHAPTER=3#C1023