I will be collecting my most commonly-used abbreviations here; if you come across something from one of my patterns that isn't on this list, feel free to Ravelry PM me @stefaniegrrr!  Thanks!

Abbreviations - Knit:
2/2 RC               slip 2 sts to CN, hold to back, k2, k2 sts from CN.
2/2 LC               slip 2 sts to CN, hold to front, K2, k2 sts from CN.
2/2 RPC            slip 2 sts to CN, hold to back, k2, p2 from CN.
2/2 LPC            slip 2 sts to CN, hold to front, p2, k2 from CN. 
Approx.             approximately
CO                     cast on
CN                     cable needle
K                        knit
K2tog                 knit two stitches together
Kf&b                  k 1 st through the front loop, then knit it again through the back loop before pulling it off the LH needle onto the RH needle
LH                      left hand
P                         purl
PM                       place marker
Sl                        slip
Ssk                      slip 1 st, slip 1 more st, knit these two together through the back of the loops using the RH needle.
St(s)                      stitch(es)
Tbl                        through back loop
RH                        right hand
Rem                       remaining
Rnd(s)                    round(s)
yo                          yarn over (1 stitch increased)

Abbreviations - Crochet
Ch              Chain
RND           Round
SC              Single Crochet
SC2TOG     Single Crochet 2 Stitches Together
ST(S)            Stitch(es)