In manga and anime, characters are often drawn sweating. This can happen for various reasons, with the sweat drops being drawn in various ways. Most of the time, the characters aren't physically sweating, the sweat merely symbolizes their mental state, like anxiety.
Top-middle: Hayate no Gotoku! ハヤテのごとく！ (Episode 11)
Top-right: Full Metal Panic, フルメタル・パニック！ (Episode 9)
Bottom-left: Nichijou 日常 (Episode 1)
Bottom-middle: Inu x Boku SS, 妖狐×僕ＳＳ (Episode 4)
Bottom-right: Bonobono ぼのぼの (1995) (Episode 1)