I've been using Piano Marvel (www.pianomarvel.com). One of the sections on it is a sight reading test, and I have been taking one (almost) every day since last December when I first got the trial (1st 30 days is free - you have to pay after that). Of course it only works if you have a digital piano
Each test starts at a simple level, and depending on your score, you get a harder piece to try, if you do well you get an even harder piece and so on. If you do poorly, you are then given a simpler piece, which you have to do well on to get any increase in score at all. Piano Marvel has a generic feature for all exercises (there are method and technique sections as well as sight reading) where it scores you a percentage of notes you get right at the right pitch and time. You have to continue to score over 80%. Three strikes (scores under 80%) and the test stops and that gives you a score for the day.
Initially my sight reading test scores were abysmal (in the low 300s), but they have slowly improved to where they are regularly over 473 (Piano Marvels boundary between beginner and intermediate) and have once or twice gone above the Intermediate/Advanced boundary.
HOWEVER, comparing the sort of music I am playing in these tests as I get to the point where I am not scoring over 80% to the ABRSM Specimen Sight Reading tests for Grade 5 (which I bought but not done many of because they are too hard for me) I would say Piano Marvel is a lot simpler.
There are some what Piano Marvel call bootcamps for sight reading (these are in the Library Section of Piano Marvel) and a while back I did take one for sight reading Hymns and started another called "Classical" and it did improve my sight reading skills. I haven't had much time on the bootcamps lately because there have so many other challenges in the Method and Technique sections, but that is another story altogether.