Ballots in open-list PR, at least if they are paper ballots, are sometimes rather complex.
Steven Taylor offers a look at a re-design of the ballot for the upcoming municipal elections in Colombia, which might help with some problems of voter confusion seen in previous municipal and congressional elections since the list-PR system was adopted in 2003. (See Steven’s links to previous posts in which he discussed these problems.)
In Colombia, parties have the option of presenting either a closed or an open list, although a very large majority of lists are open. Voters must make a party choice and then, if they choose and their party allows, may mark a candidate preference.