Equivalent Fractions by NCTM Illuminations. Use the Square setting. If you get stuck, try copying the picture exactly, then sliding one green slider. After you match the fractions the app generates, try "Build Your Own" with 3/4, 2/5, 1/6, 2/3, 5/8, 4/12, and other fractions of your choice.
Melvin's Make a Match. Match equivalent fractions from pictures, etc. If the pieces look too small, hover over them to see a bigger copy.
Fraction Flag. Choose the proportions to represent some colors in a flag (for example, half one color and half another color), then design a flag that has those proportions. The design is limited to halves and quarters (fourths).
Sizes of Fractions
Dolphin Race (Comparing Fraction Size) is goofy, but it's a crowd favorite among sixth graders. It feels competitive, but gives you time to think, and there are two different levels.
Fractions Ordering Game asks you to place fractions in order of size. If you get some wrong, it gives you another chance to rearrange those. There are three levels of difficulty.
Fraction Game by NCTM Illuminations. Use equivalent fractions and estimation of fraction sizes to "play" fraction cards on fraction number lines. (Read the instructions first.) Play several times. How few cards can you use? What are good strategies to reduce the number of cards you use?
Fraction Feud on Calculation Nation (log in as guest, or at home you can set up an account) is fun for fraction masters who like strategy games. Make a fraction SMALLER or LARGER than the one you’re “jousting” against. Try to figure out which cards are generally best to use or keep for later.
Other Fractions Activities
Sheppard Software Fraction Games include a lot of "Splat" and "Mathman" (Pacman-like) games about improper fractions and mixed numbers, equivalent fractions, adding fractions, subtracting fractions, and a LOT of other choices.
Fractions Activities on NRICH. This British math (they call it "maths") site has a lot of incredibly creative games and lessons, including challenging fractions activities. If fractions are easy for you, go here and explore, and let me know which things are interesting!
IXL Fractions Activities is a for-profit site, but it lets non-members practice a few minutes. The activities are quite repetitive, but you may like them as a sort of quiz/self-check.
Speedway (Adding Fractions) on Math Playground is good mental math practice for adding and reducing fractions, if you're already comfortable with that. Speed is a big part of this one, so make another choice if that stresses you out.
With Fraction/Decimal/Percent Jeopardy and/or Decention, you can quiz yourself on converting between equivalent fractions, decimals, and percentages. Note: in Jeopardy, use "0.3..." for 0.3 with a bar (repeating decimal). I'm not totally crazy about these activities, because I don't think it's useful to think of percents without a context (percent of what?), but it's true that 3 tenths of something is 30% of that thing, so... do it if it works for you.
Do you know other online fractions activities you want to recommend? If so, please leave a comment about them.