There's a little marking tool sold by Sally Schneider called a Gizmo that will let you make 2.5 inch HST from 2.5 inch strips. I think you can use it on strips up to about 5 or 6 inches wide.

My favorite way is to make 8 at a time.
For 2 1/2 in use two 6 in sqs. draw an x and sew down both sides of lines. Cut in half both ways then on lines. Square up slightly. I like Quilt In A Days triangle sq up ruler as it only requires one cut.
For larger sizes just add 2 in to squares. 8 in for 3 1/2.
And no bias edges.

Here's a way to do them without using a ruler.
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Check out Rhonda's tutorials on this board. She shows many efficient methods with lots of photos and examples.

I have Kimberley Einmo flying geese ruler that also cuts triangles.

I use the Joid Barrows Square in a Square ruler. Love it , you can make 4 half square triangles a one time. You can find it at: www.squareinasquare.com , you might want to at least check it out.

My favorite way is to make the strips 1/8 to 1/4 inch larger than the HST is want (plus the seam allowance). In other words, I want 2" finished half squares. I cut my strips into 3" size (2 plus 7/8 seam plus 1/8 overage). Then cut them into 3 inch squares. I use a product called Stitching Lines to mark sewing lines, sew and cut. Set seam, press square and trim down with Bloc-Lock ruler. Perfect every time. Little more trouble but perfect for Edyta Sitar quilts some of which are all HSTs.

I love Deb Tucker's "Tucker Trimmer."

http://www.studio180design.net/videos/?id=9

boy, so many options. off to try several and see what works best for me. I have several rulers and it helps knowing the proper math!

Wright's EZ Angle is my absolute "go to" when i want to use a ruler to cut my HST from strips the same size as i'm using for the squares. it's easy. it's accurate. it's not as expensive as a lot of the other specialty rulers on the market.