Kanji numbers.

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Cards

ゼロ/れい 0 (zero/rei)  
1 (ichi)  
2 (ni)  
3 (san)  
4 (yon/shi)  
5 (go)  
6 (roku)  
7 (nana/shichi)  
8 (hachi)  
9 (kyuu/ku)  
10 (juu)  
十一 11 (juuichi)  
二十 20 (nijuu)  
100 (hyaku)  
百一 101 (hyakuichi)  
五百 500 (gohyaku)  
1,000 (sen)  
10,000 (man)  
二千 1,000 (nisen)  
七万 70,000 (nanaman)  
100,000,000 (oku)  
1,000,000,000,000 (chou)  

Quisition is a browser-based flashcard system that repeats old cards and introduces new ones at optimal time intervals. You can create your own card packs or use those developed by others.