Current

Halloween Sidebar
Received by user on Neopets during October 31st 11:59 NST via Random Event.
Note: Was also received by the winners of Neopets' First Pumpkin Carving Random Contest.

Valentine’s Day Sidebar
Awarded if you send or "receive" a Mysterious Valentine's Day Card.
Note: The card is not retrievable by either the sender or recipient.

Space Faerie Premium
Refer five people on Neopets to the Premium Plan who proceed to make at least one payment; must be a member of Neopets Premium yourself.

Retired

Tale of Woe Sidebar
Awarded to everyone who collected plot prizes after participating in the Tale of Woe plot.
-Liz

Cyodrake’s Gaze Mini-Plot Sidebar
You needed to have correctly guessed two or more answers in the Cyodrake's Gaze Side Plot.

Altador Mini-Plot Sidebar
You needed to complete the Altador Side Plot.

Altador Cup Sidebar
You needed to have participated in the Altador Cup.

Qasalan Lost Desert Sidebar
You needed to complete a minimum of one step in the Lost Desert Plot.

Curse of Maraqua Sidebar
You needed to participate in the Maraquan War on either side and spent a minimum of one point on prizes.

Winter Sidebar
Received from a Random Event in Terror Mountain during 2005; it was also available in the same year from the Advent Calendar on December 31st.

Other

Birthday Sidebar
Obtainable only on two days of the year: Neopets' birthday or the day you set as your birthday while creating your account. This Sidebar can not be permanently unlocked.

Special thanks to Sploosh, who helped with much of this; re-written and formatted by Cash. Pictures of all Sidebars would be very appreciated.