The British airmen from 'Allo 'Allo! remain upbeat no matter how many times their attempts to get home are foiled.
Basi from the Nigerian TV show Basi And Company is remarkably cheerful for a broke, unemployed (and unemployable) man living in a one-room apartment (which he insists on calling "Basi's Palace"). Basi's landlady wants him out, but he's so sunny that nothing she does, including taking his only mattress, can get him to leave.
Budoy, from the 2011 Filipino drama of the same name, is an interesting Rare Male Example. Interesting in that, while he does have the sunny disposition of the pollyanna, this one might actually be partly due to the fact that he is suffering from Angelman Syndrome, a mental disorder that does create the happy and childish personality that he exhibits.
Annie in Community. Pollyanna was even used as a nickname by Jeff, when Annie's attempt to help Pierce with his ex-step-daughter backfires.
Ted from How I Met Your Mother, whose unwavering (and often irrational) amount of optimism and belief in love and friendship persists through a ridiculous amount of cynicism-inducing crap. Especially true in the field of romance, including a high school girlfriend who abandoned him immediately after tricking him into giving up his virginity for her, a college girlfriend who cheated on him constantly, having to break up with Robin even though he was in love with her, and a fiancee who left him at the altar. Oh, and he fully believed he could make it rain through The Power of Love, and succeeded. When he finally expresses a measure of pessimism in the season 7 premiere, it's a pretty disquieting moment.
Kamen Rider Den-O: Nogami Airi (the main hero's sister) suffers from selective amnesia. Apparently it has the side-effect of leaving her in a permanent state of semi-creepy, almost Rain Man-like cheerfulness.
Mara David, from the Filipino soap Mara Clara definitely fits into this to a T. In fact, she can be considered as the single biggest reason why this trope has become a staple of Pinoy teleseryes. While the girl does cry buckets, she can still remain happy even with all the torment that Clara (the second titular character) throws at her. Even when it was revealed that she and Clara were switched at birth, Mara still has her sunny disposition intact.
Subverted in the 2011 remake of the series. Mara actually does break (and occasionally snaps), becoming more cynical in the latter part of the story.
There was a 1989 made-for-TV Race Lifted (black) remake of Pollyanna, called Polly. Near the climax, the main character falls out of a tree, is seriously hurt, and is morose from there on out. By this point, she's cheered the town up so much that they try and get her to cheer up, by creating a "Polly Day". It works.
Something very similar to that happens in the original, too.
Dr. Molly Clock (Heather Graham) in Scrubs. Drs. Cox and Kelso decide to teach her that we live in a World Half Empty.
Erin on The Office. She's Pam's replacement as receptionist at Dunder Mifflin. She's more upbeat and cheerful than her co-workers and actually seems to enjoy working for Michael(but It could be because she's new). She somehow manages to find the positive in Michael lying to a group of underprivileged teens about paying their college tuitions in "Scott's Tots". We also learn in "Koi Pond" that she's an orphan.
George the Loner (played by Mark McKinney) in a Kids in the Hall sketch. He's always smiling despite the fact that he doesn't have any friends and he often gets drunk. When he stubbed his toe and no one told him to go to the hospital, which resulted in his toe becoming infected and his leg going numb.
The teacher in the Mr. Show sketch "The Chip-on-the-Shoulder Club." All the kids are jaded and none of them want to be there, but the teacher is still determined to get the students to be motivated about something (school, their future) even when all of them are jerkasses to him.
The ditzy Nymphs from the ''Charmed episode; "Nymphs Just Wanna Have Fun".