There was (maybe still is, but I cannot find it now) a list of the enabled busybox commands in FW 1.9.5 or so.
Spoiler:
Currently defined functions:
[, [[, acpid, addgroup, adduser, adjtimex, arp, arping, ash, awk, basename, beep,
blkid, bootchartd, brctl, bunzip2, bzcat, bzip2, cal, cat, catv, chat, chattr,
chgrp, chmod, chown, chpasswd, chpst, chroot, chrt, chvt, cksum, clear, cmp,
comm, cp, cpio, crond, crontab, cryptpw, cttyhack, cut, date, dc, dd, deallocvt,
delgroup, deluser, depmod, devmem, df, dhcprelay, diff, dirname, dmesg, dnsd,
dnsdomainname, dos2unix, du, dumpkmap, dumpleases, echo, ed, egrep, eject, env,
envdir, envuidgid, ether-wake, expand, expr, fakeidentd, false, fbset, fbsplash,
fdflush, fdformat, fdisk, fgconsole, fgrep, find, findfs, flock, fold, free,
freeramdisk, fsck, fsck.minix, fsync, ftpd, ftpget, ftpput, fuser, getopt, getty,
grep, gunzip, gzip, halt, hd, hdparm, head, hexdump, hostid, hostname, httpd,
hush, hwclock, id, ifconfig, ifdown, ifenslave, ifplugd, ifup, inetd, init,
insmod, install, ionice, ip, ipaddr, ipcalc, ipcrm, ipcs, iplink, iproute,
iprule, iptunnel, kbd_mode, kill, killall, killall5, klogd, last, length, less,
linux32, linux64, linuxrc, ln, loadfont, loadkmap, logger, login, logname,
logread, losetup, lpd, lpq, lpr, ls, lsattr, lsmod, lspci, lsusb, lzcat, lzma,
lzop, lzopcat, makedevs, makemime, man, md5sum, mdev, mesg, microcom, mkdir,
mkdosfs, mke2fs, mkfifo, mkfs.ext2, mkfs.ext3, mkfs.minix, mkfs.vfat, mknod,
mkpasswd, mkswap, mktemp, modinfo, modprobe, more, mount, mountpoint, mt, mv,
nameif, nc, netstat, nice, nmeter, nohup, nslookup, ntpd, od, openvt, passwd,
patch, pgrep, pidof, ping, ping6, pipe_progress, pivot_root, pkill, popmaildir,
poweroff, printenv, printf, ps, pscan, pwd, raidautorun, rdate, rdev, readahead,
readlink, readprofile, realpath, reboot, reformime, renice, reset, resize, rev,
rm, rmdir, rmmod, route, rpm, rpm2cpio, rtcwake, run-parts, runlevel, runsv,
runsvdir, rx, script, scriptreplay, sed, sendmail, seq, setarch, setconsole,
setfont, setkeycodes, setlogcons, setsid, setuidgid, sh, sha1sum, sha256sum,
sha512sum, showkey, slattach, sleep, smemcap, softlimit, sort, split,
start-stop-daemon, stat, strings, stty, su, sulogin, sum, sv, svlogd, swapoff,
swapon, switch_root, sync, sysctl, syslogd, tac, tail, tar, tcpsvd, tee, telnet,
telnetd, test, tftp, tftpd, time, timeout, top, touch, tr, traceroute,
traceroute6, true, tty, ttysize, tunctl, udhcpc, udhcpd, udpsvd, umount, uname,
unexpand, uniq, unix2dos, unlzma, unlzop, unxz, unzip, uptime, usleep, uudecode,
uuencode, vconfig, vi, vlock, volname, wall, watch, watchdog, wc, wget, which,
who, whoami, xargs, xz, xzcat, yes, zcat, zcip