This one I use daily without thinking about it - it shares my bash history between screens and sessions on a given machine and extends it (grabbed from some corner of the internet):

    # shares history between tabs with prompt_command
    HISTSIZE=9000
    HISTFILESIZE=$HISTSIZE
    HISTCONTROL=ignorespace:ignoredups

    _bash_history_sync() {
      builtin history -a         #1 appends command to histfile
      HISTFILESIZE=$HISTSIZE     #2 sets max size
      builtin history -c         #3 clears current session history
      builtin history -r         #4 reads the updated histfile and makes that the current history
    }

    history() {                  #5 overrides build in history to sync it before display
      _bash_history_sync
      builtin history "$@"
    }

    PROMPT_COMMAND=_bash_history_sync

Sometimes I forget the name of git branches I've worked on lately. This displays them with dates, most recent at the bottom:

    git_branch_dates() {
      git for-each-ref --sort=authordate --format '%(authordate:iso) %(align:left,25)%(refname:short)%(end) %(subject)' refs/heads
    }

Because I hate typing:

    # for rails migrations and keeping test updated
    alias migrate="rake db:migrate && RAILS_ENV=test rake db:migrate"

Something I should use more to track things I've done lately - this opens up vim at the end of a file in your home dir called did.txt, appends the date and puts cursor at the last line:

    # didfile setup
    alias did="vim +'normal Go' +'r!date' ~/did.txt"

Because sometimes you want to sudo an alias:

    # trailing space in value expands aliases that come after, see: http://www.linuxcommand.org/lc3_man_pages/aliash.html
    alias sudo='sudo '

And displaying the git branch in the prompt:

    # this gets the git branch to display when I cd into a git repo
    parse_git_branch() {
        git branch 2> /dev/null | sed -e '/^[^*]/d' -e 's/* \(.*\)/ (\1)/'
    }

    export PS1="\[\033[36m\]\u\[\033[m\]@\[\033[32m\]\h:\[\033[33;1m\]\w\[\033[m\]\[\033[32m\]\$(parse_git_branch)\[\033[00m\]\$ "

Because I often want to check recent files:

    alias llth='ls -hFlat | head'