Common extensions on different aspects of the language, basic types, strings, collections, files, control structures, exceptions, logging, et cetera.
Here are some highlights pointed out but you might want to visit the KDoc for a more complete overview of the provided functionality.
fun Long.timify(format: MsTimification): String
fun <IN, OUT> IN?.nullOrWith(wither: (IN) -> OUT): OUT?
fun Throwable.stackTraceAsString(): String
fun Array<StackTraceElement>.formatted(): List<String>
fun Int.isBetweenInclusive(lower: Int, upper: Int): Boolean
fun Int.forEach(code: () -> Unit)
fun Double.format(digits: Int): String
fun Double.toSeconds(digits: Int = 3, suffix: String = " secs"): String
fun String.removePreAndSuffix(search: String)
fun String.containsAll(vararg substrings: String, ignoreCase: Boolean = false)
operator fun StringBuilder.plusAssign(char: Char)
fun String.saveToFile(target: File)
fun <K, V> MutableMap<K, V>.put(pair: Pair<K, V>): V?
fun <K, V> mapsOf(vararg maps: Map<K, V>): Map<K, V>
fun List<Any>.prettyPrint(prefix: String, joiner: String)
fun <K, V> Iterable<Pair<K, V>>.toMutableMap()
fun tempFile(name: String): File
fun File.verifyExists(): File
fun File.move(target: File)
fun File.touch()