Crochet for knitters

RS of sample - includes edge samples, some bobbles,
surface slip stitch, and seams.  Also looks like a goofy person.
As practical as knitting is for all kinds of things, sometimes crochet is faster and easier for a few details.  Most common is the crab stitch edge to finish a knitted garment.  This sample has a number of examples of crab stitch edging.

Crochet can also be useful for seams and for embellishment.  When adding crochet to a knit fabric, choose a hook that is small enough to work comfortably into your knit fabric and is also big enough for your yarn.  F/G/4mm is a common size to use with worsted or DK weight yarns.  

Note that yarnovers in knitting are counter-clockwise Q, while yarnovers in crochet are clockwise P.  If that sounds confusing, just remember to wrap the yarn the normal way when knitting and the other way when crocheting. 

Crab Stitch
Edges:  Crab stitch is a name for doing single crochet from left to right, so the stitch is twisted.  If you haven't tried it before, there is an existential learning curve, and you may need to keep at it for a dozen or more stitches before it starts to make sense.  It really is just single crocheting from left to right, but the trick is for each stitch to be twisted.  A variation is to start a single crochet the normal way (from right to left), but rotate (twirl the hook around one time) to twist the stitch before doing the final yarnover and pull through.   

Lite Crab Stitch 
Crab stitch makes a very solid corded edge.  For something just a little lighter, consider lite crab stitch:  alternate a crab stitch with a chain stitch.

You can crab stitch directly into the edge of the knitted fabric, but sometimes the stitching can look different depending on whether you're stitching into row ends or into stitches.  In that case, it can be useful to start with a setup row, using the same yarn as the fabric, to make a consistent base for the contrast edging.
Shell Stitch border - very traditional

Other traditional edges include simply stitching a single row of a pattern stitch, like Shell, or Open Shell, or Crazy Stitch.  Including Picots makes the row look fancier by adding little pointy bits.  
Shell Stitch with picots

For a flared edge, consider - instead of binding off - doing a (slip stitch to bind next stitch off the left needle, then chain 1) all the way across.  To add a flared edge to an edge other than the last row, pick up stitches along the desired edge, stockinette a few rows (enough to make a ruffling fabric), then do the (slip stitch, chain) combination to bind off.  The massive increase of (a) switching from knit to crochet, and (b) adding the chain stitches will make the edge ruffle out a bit.  Working into the live stitches makes the knitted fabric shape itself into the expanded edge.

Seams:  Three-needle bind-off, mattress stitch,  and Kitchener / grafting are such perfect ways to join fabrics in knitting, you'd think crochet wouldn't have anything to add.  (An aside:  Did you ever notice that mattress is just like grafting, only tighter and on row ends rather than in stitches?)  If your yarn is really textured or delicate (like mohair), it may not be suitable for using with a yarn needle, and the three-needle bind-off is only for, well, binding off, which still leaves other seams to do.  

I learned the (slip stitch 1, chain 1) seam when altering a store-bought knitted sweater where the sleeves were too long and could not be shortened at the wrist.  Taking the sleeves out and shortening the sleeve cap worked just fine -- and the armhole seam had been joined that way.  Here, the seam is worked in a contrasting yarn, and you can see it on the right side.  It blends in much more if you use the same yarn as the fabric. 

Other seams make sense with projects like joining motifs.  In a pattern the instructions may simply say to whip-stitch, or just 'join'.  I generally don't care for whip-stitching seams because it tends to come apart easiest.  If the motifs, joined together, are coming out too small or too dense, it can make sense to join them with a lacy zigzag crochet that adds more fabric as well as 'give' between the squares.

Embellishments:  Even the most sensible knitter can have an embellishment emergency, and a few very simple concept tools in your kit can come in handy.  The simplest basic flower and leaf is easy, quick, and makes you look clever.  Other common embellishments include the Bobble (either worked in or added after), and Slip Stitch (as an alternative to Duplicate Stitch, and it is handy for designs that don't follow the grid-like structure of the knitted fabric).